The anemometers in use today commonly employ a hot wire technology consisting of an electrically heated, fine platinum wire that is immersed into a fluid flow stream. As the velocity of the flow stream increases, the rate of heat transfer from the heated wire to the flow stream increases. Thus, a cooling effect on the wire occurs, causing its electrical resistance to change. In a constant-current anemometer, the fluid velocity is determined from a measurement of the resulting change in wire resistance. In a constant-resistance anemometer, fluid velocity is determined from the current needed to maintain a constant wire temperature and, thus, the resistance.
Hot-wire technology, however, has several drawbacks. The fragility of the sensors can cause loss of calibration or physical damage if a unit is accidentally dropped. Also, the wire may oxidize over time, changing the calibration of the sensors. Further, because the hot-wire signal is weak, a small drift over time can cause loss of calibration or inconsistent readings. When a problem occurs with hot-wire technology, the entire unit must be sent back to the factory for repair and/or recalibration, leaving the user without equipment for several days. In addition, hot-wire anemometers can be very sensitive to a small variance in position relative to the direction of the fluid flow stream.
Other anemometers have been employed using two thermistors placed on opposite sides of a heating element along a single axis. The configuration is placed in the path of a fluid stream so that there is an upstream thermistor and a downstream thermistor and each thermistor monitors the same fluid stream. A thermistor is a temperature-sensing element composed of semiconductor material that exhibits a large change in resistant proportional to a small change in temperature. When no fluid is flowing across the anemometer, a heat wave propagates in all directions from the heating element equally heating both thermistors. When fluid flows across the thermistors, more heat is transferred to the downstream thermistor causing the electrical resistance of the thermistors to respond accordingly. Thus the direction of the flow stream can be calculated by determining which thermistor is warmer. The magnitude of the flow rate can be calculated based on the temperature differential of the thermistors.
Using thermistors in an anemometer provides several advantages over hot-wire technology. The thermistor signal may be as much as 1000 times larger than that of a hot-wire electrode. The sensor will not change calibration significantly and it is resistant to shock and vibration. The thermistor can be dropped without losing its calibration or sustaining physical damage. A disadvantage of the anemometer configuration described above is that it requires the use of an independent heating element, which can consume significant power. It also can be difficult to completely remove noise created by the heat wave itself. Additionally, in order to achieve accurate measurements, two higher-cost thermistors must be provided.
Accordingly, it is desirable to provide an apparatus and method for measuring fluid flow direction that uses two inexpensive sensors and does not require the use of an independent heating element. Such an apparatus and method can be utilized in Continuous Positive Airway Pressure (CPAP) systems, ventilators, automotive systems, and other fluid measurement systems.